In conventional moon phase display devices, a disc bearing two images of the full moon makes one half-revolution per lunation behind a semi-circular window of a peculiar shape, illustrated for example in U.S. Pat. No. 508,467. One of the edges of the window includes two convex arcs that cut into the image of the full moon, respectively while the moon waxes and wanes. The shape of the moon image thus displayed is correct only at the beginning and at the end of the lunation (starting from the new moon), when the illuminated part has the shape of a crescent, and at the full moon. During the other phases, the image displayed has an incorrect shape, because the shape of the line of separation between the light zone and the dark zone does not conform to reality: it is curved instead of being straight at the first and last quarter, and it is bent in the wrong direction between the first and the last quarter.
Various solutions have been proposed to avoid this drawback.
A display device of the kind indicated hereinbefore in the preamble is disclosed in U.S. Pat. No. 6,507,536 and includes two partially superposed rotating discs, each bearing a dark zone limited by a curve. The rest of the upper disc is transparent to show a part of the lower disc, outside the phase of the new moon. The two discs are synchronously driven by gears. Their respective dark zones are combined behind the window to give, at each phase of the moon, an image in which the shape of the light window to give, at each phase of the moon, an image in which the shape of the light zone corresponds as far as possible to that of the moon seen from the earth. Such a device is relatively bulky in plane. At the phases where the separation line is formed by a combination of the dark zones of the two discs, one cannot always avoid the appearance of a break in said line at the place where the edges of the two dark zones intersect. Moreover, the image of the moon can only be formed in one plane, since it is formed of two parts that are offset mutually in depth in the direction of vision, and this constitutes a drawback from an aesthetical point of view.
In EP Patent Application No. 1 103 872, the moon is represented by a transparent circular disc that moves linearly in front of a dark screen having a sinuous elongated aperture. The width of the aperture varies from a maximum at the middle, corresponding to the diameter of the lunar disc, to zero at the ends. The lunar disc moves pressing against a sinuous cam surface, such that one of the edges of the aperture is tangent to the disc and the other edge forms a line of separation almost matching reality, between the light part and the dark part of the lunar disc. However, the device disclosed in this document is too bulky to be incorporated in a watch. Further, such a display is difficult to read if it is not lit from behind.